Various technologies and circuit expedients have been used to provide conferencing facilities in time division switching systems. The J. C. Moran U.S. Pat. No. 3,996,566, issued Dec. 7, 1976, discloses a switching system having a single PAM time division bus connected to all of the system port circuits. On two party calls, the port circuits of the two parties are interconnected by assigning them to the same time slot and by activating circuitry within the two port circuits so that they can exchange speech samples over the bus during each occurrence of the assigned time slot.
A single bus system of the type shown by Moran can also serve three party calls by simply assigning all three stations to the same time slot and by making the port circuit of each such station active during the assigned time slot. This permits the three stations to intercommunicate with a transmission level that, although degraded somewhat, is still acceptable. The transmission level is degraded further as more stations are added and, therefore, this expedient cannot be used on conference calls involving more than three stations.
It is known in U.S. Pat. Nos. 4,049,920 of Sept. 20, 1977 to Knollman and 3,997,730 of Dec. 14, 1976 to Stidham to use facilities, such as conference bridges and the like, for serving conference calls involving more than three stations in single bus systems of the type shown by Moran. Each conference bridge typically includes a plurality of nodes each of which is assigned to a different time slot and connected to a different conferee station during the serving of a conference call. The conference bridge circuitry processes the signals of all conferees so as to provide satisfactory transmission levels.
Single bus systems as shown by Moran are suitable only for use in small to medium size installations since the system's call serving capacity is limited by a number of factors, including the number of system time slots. For example, in a 64-time slot system, no more than 64 simultaneous calls can be served.
Increased call serving capacity has been provided in prior art time division switching systems by the use of switching facilities of greater complexity. One such prior art arrangement is embodied in the DIMENSION.RTM. 2000 PBX manufactured by the Western Electric Company, Incorporated. That system provides increased call serving capabilities by the use of a plurality of separate network modules with each module essentially being of the type shown by Moran and with the modules being interconnected by a system of voiceband links for the transmission of call message information between modules on intermodule calls.
The DIMENSION 2000 PBX is disclosed on pages 86-93 of an article entitled "Development of Electronic Tandem Service (ETS) Features for the DIMENSION PBX" by A. M. Gerrish and D. C. Opferman in an IEEE publication identified as CH1515-6/79/0000-0086$00.75.COPYRGT.1979 IEEE. Each module of the DIMENSION 2000 PBX is connected to each of the other modules by a group of voiceband links unique to two of the modules being interconnected. Thus, for a three module system having modules a, b, and c, a first link group interconnects module a and b, a second group interconnects modules a and c, and a third group interconnects modules c and a. In a similar manner, six link groups are required to connect a four module system, ten link groups are required for a five module system and so on. The formula for determining the required number of link groups is (n-1)n/2 where n is the number of modules. Each link group comprises a pluralitry of links where a single link comprises the voice channel facilities required for a single call between the two interconnected modules. Each link terminates in a link port circuit on each of the two modules it interconnects, and each link port circuit is connected to the PAM time division bus of its module in the same manner as for a station port circuit in Moran.
Intramodule calls are served by the DIMENSION 2000 PBX in essentially the same manner as shown by Moran. Two party intermodule calls are served by using any available time slot in each module and by transmitting the message information over a link path between modules at voiceband. A conference call between stations of three different modules is served by assigning three different port circuits in the calling module to a single time slot so that they are connected to the PAM bus of the calling module during each occurrence of the assigned time slot. The first of these three port circuits is the line port circuit of the calling conferee station. The second port circuit is the port circuit for the link that extends to the first of the other two modules. The third port circuit is the port circuit for the link extending to the third module involved on the call. Thus, a three party intermodule conference call is served by the calling module in a manner analagous to that of a three-party intramodule conference call in Moran in that three conferee port circuits in the calling module are all assigned to the same time slot.
Intermodule conference calls involving more than three parties are not served by the DIMENSION 2000 PBX by merely increasing the number of port circuits that are assigned to the same time slot in the calling module. The reason for this is the same as for Moran; namely, the transmission is degraded to an unacceptable level when more than three port circuits within a single module are assigned to the same time slot.
The conferencing facilities of the DIMENSION 2000 PBX include a Stidham type conference bridge in each module for the serving of conference calls involving more than three stations. The conference bridge in the calling module is used on each such call. The conferee stations in the other modules are connected over the voiceband links to the link port circuits of the calling module and each such link port circuit is connected during a different time slot to a different node of the Stidham conference bridge.
In summary, the prior art arrangements for serving conference calls are limited in their application even though they are satisfactory for the purposes for which they were designed. The Moran system, when combined with Stidham, is limited to a single bus system that is useful only in small to medium size installations. The DIMENSION 2000 PBX, when combined with the Stidham, provides increased conference call serving capabilities; however, it does so by the use of an array of interconnecting voice band links as well as the use of complex timing and network synchronization facilities.